Use your nose to sniff the oil to see how strong the fishy smell is.
If you can't detect any or little, you'll be ok to use it again
Fish can certainly flavor fry oil but if you have a batch of oil and it's still clean and you don't want to toss it you don't necessarily haveto.
I've found that a batch or two of french fries can help remove some or nearly all of the fishy flavor from oil.
Of course then the french fies have a little, "Extra flavor" but that's never stopped me from eating them anyway.